1. Field of the Disclosure
The present disclosure relates to a current sense apparatus.
2. Description of Related Art
It is desirable to be able to determine a current capacity level of a battery or an array of batteries or cells. Gas gauge (also known as fuel gauge) circuitry can be used to predict an amount of energy stored in a battery pack, or in other words its current capacity. The prediction can be conducted by measuring Coulombs of charge going into and out of the battery. This is known as Coulomb counting. This can be achieved by measuring a voltage drop across a current sense (or shunt) resistor, which can be provided either at a high side (positive) or low side (0V or ground) terminal of the battery. For low voltage batteries (for example 3V to 12V) it is not crucially important whether the current sense resistor is provided at the positive or low-side (which may be) ground (0V) terminal. However, for higher voltage batteries (for example 24V to 60V) it is more desirable to use current sensing at the 0V terminal due to the low operating voltage of most semi-conductor processes used in commercial off-the-shelf gas gauge integrated circuits. An example gas gauge for this type of battery is the TI (Texas Instruments) bq27541-v200, which has a maximum supply voltage of 24V and employs low side current sensing. The TI bq27541 provides accurate capacity estimation and is known to work with a typical battery current waveform.
Unfortunately, it is difficult to provide a current sense resistor at the 0V terminal, mainly because access to the 0V terminal may be restricted by a need to connect the battery to a common ground (for example a car chassis), or by the need to provide protection circuitry at the 0V terminal. Further, there may be separate 0V terminals used for charging and discharging of the battery. For these reasons it may not be practical to directly measure current at the 0V terminal. Additionally, there may be multiple loads connected to the positive terminal of the battery, and it may be desirable to measure the current drained by each and/or all of these.
If a battery is provided with a high side current sense resistor, a direct interface with a gas gauge which uses low side current sensing (such as the bq27541) is problematic.
According to a first aspect of the present disclosure there is provided a current sense circuit for measuring a charge level of a battery, the circuit comprising: a shunt resistor (R10) connected between a high side terminal of the battery and a load/charge terminal for connecting the battery to a load; translation circuitry arranged to produce a voltage across a pair of current sense terminals (GG_SRP, GG_SRN) in proportion to the voltage across the shunt resistor; wherein one of the current sense terminals (GG_SRP) is provided on a first current path connected at one end between the high side terminal of the battery and the shunt resistor (R10), and connected at the other end to ground, and the other of the current sense terminals (GG_SRN) is provided on a second current path connected at one end between the shunt resistor (R10) and the load/charge terminal, and connected at the other end to ground.
The first current path may comprise a first resistor (R21) via which said one of the current sense terminals (GG_SRP) is connected to ground and a second resistor (R4) and a first transistor (Q4) provided in series via which said one of the current sense terminals (GG_SRP) is connected to the high side terminal of the battery; said second current path comprises a third resistor (R20) via which said other of the current sense terminals (GG_SRN) is connected to ground and a fourth resistor (R5) and a second transistor (Q8) provided in series via which said other of the current sense terminals (GG_SRN) is connected to the load/charge terminal; the first resistor (R21) has the same resistance as the second resistor (R4), and the third resistor (R20) has the same resistance as the fourth resistor (R5); said first transistor (Q4) and said second transistor (Q8) together control the voltage drop across the second resistor (R4) and the combined voltage drop across the fourth resistor (R5) and the shunt resistor (R10) to be substantially the same.
The first transistor (Q4) and the second transistor (Q8) may be controlled by feedback circuitry, the feedback circuitry comprising: drive circuitry operable to drive the gates of the first transistor (R4) and the second transistor (Q8) to increase or decrease the voltage drop across the second resistor (R4) and to increase or decrease the combined voltage drop across the fourth resistor (R5) and the shunt resistor (R10); and an operational amplifier configured to compare the voltage drop across the second resistor (R4) with the combined voltage drop across the fourth resistor (R5) and the shunt resistor (R10), and to control the drive circuitry to drive the gates of the first transistor (Q4) and the second transistor (Q8); wherein if the voltage drop across the second resistor (R4) is greater than the combined voltage drop across the fourth resistor (R5) and the shunt resistor (R10), the feedback circuitry increases the gate voltage to the first transistor (Q4) to reduce the voltage drop across the second resistor (R4) and decreases the gate voltage to the second transistor (R8) to increase the combined voltage drop across the fourth resistor (R5) and the shunt resistor (R10), and if the voltage drop across the second resistor (R4) is less than the combined voltage drop across the fourth resistor (R5) and the shunt resistor (R10), the feedback circuitry decreases the gate voltage to the first transistor (Q4) to increase the voltage drop across the second resistor (R4) and increases the gate voltage to the second transistor (Q8) to decrease the combined voltage drop across the fourth resistor (R5) and the shunt resistor (R10).
The drive circuitry may comprise: an additional supply rail at a fixed voltage below the high side terminal of the battery; a first drive path connected at one end between the high side terminal of the battery and the shunt resistor (R10), and connected at the other end to the additional supply rail, the first drive path being operable to drive the gate of the first transistor; and a second drive path connected at one end between the shunt resistor (R10) and the load/charge terminal, and connected at the other end to the additional supply rail, the second drive path being operable to drive the gate of the second transistor.
The first drive path may comprise a fifth resistor (R8), a third transistor of a current mirror (Q7) connecting together the first drive path and the second drive path, and a sixth resistor (R17), and the second drive path comprises a seventh resistor (R11), a fourth transistor of the current mirror (Q7), a fifth transistor (Q1-A), a sixth transistor (Q1-B) and an eighth resistor.
The current sense circuit may comprise filter circuitry for low pass filtering the output of the operational amplifier.
The current sense circuit may comprise a reference path connected at one end between the shunt resistor (R10) and the load/charge terminal and connected at the other end to the additional supply rail, the reference path comprising a ninth resistor (R9) and a tenth resistor (R12), the ninth resistor (R9) and tenth resistor (R12) forming a potential divider outputting a reference voltage to the gates of each of the fifth transistor (Q1-A) and the sixth transistor (Q1-B).
A smoothed output of the operational amplifier may be asserted at the emitters of each of the fifth and sixth transistors.
The voltage drop across the first transistor and the second transistor may be controlled to maintain a constant voltage drop across the second resistor, and a constant combined voltage drop across the fourth resistor and the shunt resistor when the voltage at the high side terminal of the battery varies.